The dual blocking assay, immunoprecipitation technique and in vivo immunosuppression studies have been used to investigate the relationships between molecules on the cell surfaces of mouse thymocytes and lymphocytes. In the past year, we have been investigating the topographical relationships between mouse alloantigens recognized by monoclonal antibodies. Our results indicate that supramolecular relationships can be detected by our techniques and suggest that these relationships may have importance in cell-cell interactions during the immune response. In particular, we have studied the relationships between a number of T-lymphocyte alloantigens, including Ly 1, Ly 2, T200, Thy 1, Qa-2 and TL. Moreover, we have obtained preliminary evidence that there are molecular and/or cell surface relationships among the Qa-2 family of antigens. By treating mice from birth with monoclonal anti-Qa-2, we can suppress the development of the Qa-2 antigen as well as the development of Qa-3, Qa-5 and Qa-6.